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Often I've seen a question that can be easily solved with the implementation of a track matte. Often it isn't the creative limitations of the user, it's just that people don't know it exists. Although it's been featured in several tutorials in the AE Channels here on DMN, a simple review might clarify its use.In Photoshop, you have the ability to create layer masks which create masks based on luminance for the still image on that layer. In After Effects, the layers can contain animation. Moreover, the layers may have properties which animate as well. Because you may want your fill layer may animate in a manner separate from your mask layer, it's important to be able to keep these layers separate (rather than using a mask to simply cut out the fill).
There are a few things you need to know about Track Mattes:
The Track Matte option is found in the Switches/Modes column of the timeline.
Track Mattes work in pairs. A track matte option won't be visible if you only have one layer in your composition. It relies on one layer being the fill and another being a matte. The top layer in the pair is the matte. The bottom layer in the pair is the fill.
Track Mattes work on two separate image principles. Either based on Alpha Channel or based on luminance of the pixels within the matte layer. You can opt to use the inverted data of these two options by choosing one of the companion pull-down selections. (Of course you can turn if off by selecting "None")
Alpha Channel Track Matte
Using an alpha matte in Track Matte mode allows the bottom layer (the fill) to use the alpha channel of the layer above it. The alpha channel can be interpreted from the transparency value of an imported still image, an embedded channel in a QuickTime movie, or even a mask created on the top layer made to create an alpha.
In this example, I have a merged Photoshop document, yellow with blue dots. (click for larger version to download)
Import the pattern into After Effects, and place the image in a new composition (320 x 240 px).
Next, create a white solid and make a new mask shape in the outline of a star.
Now that you have a layer above the dots image, you will also have the option to select a Track Matte option. Using the pull-down menu, select the Alpha Matte option. Note: After Effects has automatically made your matte layer invisible by turning the eyeball off in the visibility column. Now the top layer cuts out the bottom layer.
Next, animate the star layer's rotation value. Animate the dots.psd position. Notice that the two animations exist separately, because they're on two different layers.
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Source:Ko Maruyama.
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